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Case Study: Amplifier Input Impedance Testing in B² Spice

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Measuring Input Impedance
An amplifier’ input impedance can be an important piece of information, as many amplifiers are often cascaded and one amplifier’s input impedance becomes another amplifier’s load impedance. Unintended audio band low-pass filters can be created if one amplifier’s coupling capacitor sees too little a load impedance.

An amplifier’s input can often be determined by mere visual inspection of the schematic. For example, an op-amp or vacuum tube circuit with a 47k resistor from input to ground has an input impedance of 47k. But not always. If the tube circuit uses a grounded-grid input stage, the input impedance can be quite low or if a current-feedback op-amp is used and its inverting input is used, then it too will offer an extremely low input impedance.

Measuring the input impedance is easy enough. The first step is to place a current source in series with ground and the amplifier’s input. The source’s current value should be set to some innocuous value, such as 1µA and 1kHz. Next we perform a Transient Sweep test and plot the ensuing sine wave at the amplifier’s input. As the waveform is unlikely to be perfectly symmetrical, we measure the peak-to-peak value and divide this value by twice the current (2µA) to get the average input impedance at 1kHz.

Measuring Output Impedance
An amplifier’s output impedance and the load impedance define the amplifier’s damping factor, as the damping factor is but the ratio of load impedance divided by the output impedance, thus the ridiculously high values of 400 or even 1000.

Measuring the output impedance evolves the strategy as the measuring of the input impedance. Once again a current source is added to the circuit with a low current value, say 1mA, and once again1kHz is specified. This time the source is placed between ground and the amplifier’s output. A Transient Sweep test is run and sine wave created at the amplifier’s output is plotted. Then the peak-to-peak output voltage value is divided by twice the current value, which results in the output impedance in ohms.

For most, this will be all that is needed, but for a few power users, this information does not go far enough. More information will be desired. For example, what is the output impedance of across the frequency spectrum?

Creating a frequency against output impedance requires setting the current source’s transient properties to DC, with 0 volts as an offset, and then setting its Magnitude to 1mA under the Small Signal AC and Distortion tab.

Then we need to run a Small Signal AC Sweep test. Once again we will be given many plots, but not quite the one we want. So the next step is to turn 0ff all the existing plots and to define a new plot:

This plot takes the absolute of output voltage against 1000, the inverse of 1mA. Now the plot displays the output impedance in ohms.